Thread: trouble shooting a fuel gauge

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I installed a Autometer(0 to 90 ohm) sending unit in my poly gas tank. I ran 2 wires(one from the sending unit terminal & one from the sending unit ground) to the gauge. There is also a hot wire from ign to gauge.

The gauge reads below empty....if I put second wire from the sending unit ground to frame It reads above full.

The resistance between the 2 sending unit terminals reads 60 ohms.

What now.....Bad gauge? everything is new.

Don

Looking at your picture, it appears you riveted the sender to the hex plate, leaving one screw for the ground? Did you follow the AutoMeter instructions and put the little o-ring under each of the six screws that hold the level sensor in place or did that plate/sender come with the tank? If you used the o-rings try taking the o-ring off of the screw with the black wire so it can make solid contact with the flange of the sensor. Second, you say you ran a wire from the sending unit to ground? I don't see that chassis ground here? It should be to the same screw as the black wire shown. The o-ring is what was causing mine not to work on initial install, and you need a good solid ground to the sensor flange, and for the flange to gauge ground.

The real problem may be your sender. The sending unit is a variable resistor, so if yours is a 0-90 ohm you should see it go through that range as you move the float from high to low. AutoMeter has two different lines of gauges, and most of their street rod type dials seem to have their 240 - 33 ohm fuel level gauges, which is the resistance range of the AutoMeter universal sender from what I see, and from my set. Did you buy your gauge set complete with the sending units, or were they separate? Sounds like maybe a mis-match in range?

The gauge and the sending unit are matched, correct. The 90 ohm sender is GM.

As to wiring, hot too one #1 terminal of guage, from the other terminal #2 to the sending unit center lug. Normally that's all it takes, however because you are using the polly tank.

You need to run a wire from the sender mounting plate to ground. Looking at the photo I would say the black wire too ground. That red should come from the fuel gauge

What is the rated voltage for the guage?

Most gauges require a voltage reducer.

Getting a different reading by adding a ground indicates your ground isn't making a good connection.

Originally Posted by firebird77clone

What is the rated voltage for the guage?

Most gauges require a voltage reducer.

Getting a different reading by adding a ground indicates your ground isn't making a good connection.

Many OEM gauges use a voltage reducer, but Don said he's got AutoMeter which take a 12V feed. Also, Pepi's right on that black wire - it's likely the chassis ground. My AutoMeter fuel level gauge has three connections, 1) switched 12V, 2) input from the sending unit and 3) dedicated ground. Likewise the sending unit has one lug connection for the output, assuming a grounded tank. With a poly tank or fiberglass car a sending unit ground completes the circuit.

I'm betting it's an AutoMeter standard gauge wanting 240-33 ohm input, but a 0-90 ohm sender.

I called Summit to check what I ordered.....it was the 0 to 90 ohm gauge, there was no sending unit on this order.

I do not remember where the sending unit came from......I'll remove it & check the resistance by moving the arm up down
& report back.

Thanks Don

So you verified the wiring ?

Rereading your post
I installed a Autometer(0 to 90 ohm) sending unit in my poly gas tank. I ran 2 wires(one from the sending unit terminal & one from the sending unit ground) to the gauge. There is also a hot wire from ign to gauge.

The gauge reads below empty....if I put second wire from the sending unit ground to frame It reads above full.
If this is the red wire that you grounded then it is working

Just an FYI here you can also read the battery 12v on that same wire.
1. Wire lifted, + meter lead attached
2. - meter lead to ground
3. 12 V



The resistance between the 2 sending unit terminals reads 60 ohms.

Is the tank empty? Sending unit arm resting on bottom of tank. If it were hanging should read 90 ohms


The document is a drawing of the wiring, the drawing shows the ground wired back to the gauge. That is not necessary, any metal of the car and frame are a ground, sheet metal screw and lug on wire will fill the bill.

Document1.doc

One last comment if the wiring is correct; the components are matched and you are reading below empty on the gauge . You need to switch the wires at the back of the gauge, the polarity is backwards.

Be sure to let us know how you make out

I can't see the picture Pepi posted (it says that an additional program & decompressor are required to open the picture in the MSWord file), but one thing that jumped out at me reading Pepi's post is Don's statement

I ran 2 wires(one from the sending unit terminal & one from the sending unit ground) to the gauge. There is also a hot wire from ign to gauge.

As pointed out the ground connection at the gauge and the ground connection at the tank are not to be connected together via insulated wire, but are a chassis ground to complete the circuit from the ignition power, out through the sending unit and back to the gauge input. If you simply connect the gauge (-) to the sending unit (-) you have no ground with the poly tank, right?

The tank has about 12 gal in it & is reading 60 ohms.......I will ground the gauge to the metal dash & ground the sending unit to the trunk sheet metal removing the blk wire I have going form gauge to sending unit.

I am having chest pains & am waiting a heart valve replacement so it may be a while before I can play with it........thanks to all for the great info.

Don